Electric motor vehicles are generally provided with an auxiliary power supply for supplying electric power to, for example, lighting and air conditioning systems therein. The loads (for example, the lighting and air conditioning systems) on the auxiliary power supply generate heat (loss), regardless of vehicle speed, in a main circuit element of the auxiliary power supply. Accordingly, cooling of the main circuit element is necessary. With respect to the cooling system for the main circuit element of the auxiliary power supply, a self ventilation type using ram wind induced by a running vehicle (running-vehicle wind) and a forced wind cooling type using a fan are applicable.
Because the strength of the running-vehicle wind depends on the vehicle speed, the cooling ability when a self ventilation type is adopted decreases with slowing of the vehicle speed, and becomes the lowest during stopping of the vehicle, while the running-vehicle wind cannot be expected. Accordingly, in order to secure the cooling ability under the worst condition, cooling of the main circuit element of the auxiliary power supply is designed in a windless state during the vehicle stop as a design condition.
On the other hand, regarding a configuration according to an electric power conversion system of an AC electric motor vehicle, in a case of a configuration in which the auxiliary power supply is connected to an intermediate link connection of a main conversion unit (converter-inverter), input voltage of the auxiliary power supply is an intermediate link voltage in the main conversion unit. Because the voltage generally depends on a main motor (motor for driving the vehicle) as a load on the main conversion unit, the voltage is set at a higher level, and thereby loss generated in the main circuit element tends to become larger than that in a configuration in which electric power is directly obtained from a main transformer.
Conventionally, as a measure for reducing the loss generated in the main circuit element of the electric power conversion system, as disclosed in Patent Document 1, a method has been known in which a DC-AC conversion means (inverter) and an AC-DC conversion means (converter) are controlled in response to load electric power, or input electric power, etc., as an example, input voltage of the inverter is changed to be high or low (for example, refer to Patent Document 1).